The present invention relates to a pressure roller and a heat-fixing device including such a pressure roller for use in an electrophotographic image forming apparatus. More specifically, the present invention relates to a roller for nipping a recording material for conveying the recording material, and particularly a pressure roller used in a heat-fixing device for permanently fixing a toner image onto a recording material in a recording system according to, e.g., electrophotography or electrostatic recording.
Many of conventional electrophotographic copying machines and printers have adopted, as fixing means, those of the heat-fixing type including a contact-heating hot roller fixing device exhibiting good heat efficiency and safeness and a film-heating type fixing devices requiring less energy consumption.
A heat-fixing device of the hot roller type includes a heating roller (fixing roller) as a rotating member for heating and an elastic pressure roller as a rotating member for pressurization pressed against the heating roller as basic members. A pair of the rollers are rotated, and a recording material (such as transfer paper, electrostatic recording paper, electrofax paper or printing paper) carrying a yet-unfixed toner image conveyed to a pressure nip (fixing nip) between the rollers is nipped and passed between the rollers, whereby the toner image is heat-fixed to form a permanently fixed image on the recording material surface under the action of a heat from the heating roller and a pressure at the pressing nip.
A typical of the elastic pressure roller used for the above purpose comprises a solid or hollow cylindrical core metal or metal substrate, an elastic layer formed on the metal substrate, and a toner release layer of a fluorine-containing resin formed on the outer peripheral surface of the elastic layer. In order to ensure a sufficient contact area between the recording material and the heating roller, the pressure roller is required to exhibit a sufficient elasticity so that the elastic layer is formed in a relatively large thickness. The elastic layer frequently comprises silicone rubber in view of the heat resistance.
Fixing devices of the film-heating type are disclosed, e.g., in Japanese Laid-Open Patent Application (JP-A) 63-313182, JP-A 2-157878, JP-A 4-44075 to 4-44083 and JP-A 4-204980 to 4-204984. In a typical film-heating type fixing device, a heat-resistant film (fixing film) as a rotating member for heating is pressed against a heating member (heat-generating member) while being moved in contact with the heat-generating member by the action of a rotation member (elastic roller) for pressurization, and a recording material carrying a yet-unfixed toner image is conveyed to a pressure nip formed between the heating member and the elastic roller pressed against each other via the heat-resistant film and moved together with the heat-resistant film, thereby fixing the yet-unfixed toner image to provide a permanently fixed image on the recording material under the action of a heat imparted from the heating member via the heat-resistant film and a pressure at the pressure nip.
Such a film-heating type fixing device allows economization of electricity and shortening of a waiting time (i.e., an improved quick-start performance) as the heating member can comprise a low-heat capacity linear heating member and the heat-resistant film comprises a film of a low-heat capacity.
As fixing devices of this type, there are known those according to a scheme wherein a drive roller is disposed in contact with an inner surface of the fixing film to drive the fixing film under tension and also those according to a scheme wherein a fixing film is wound loosely about a film guide and a rotating member for pressurization is primarily driven to drive the fixing film in rotation following the rotation of the rotating member. In recent years, the latter pressure rotation member (pressure roller) drive-type devices are more frequently adopted in view of a smaller number of parts required therein.
The elastic roller used for the above purpose may frequently comprise a solid or hollow core metal or metal substrate, a silicone rubber elastic layer or a silicone sponge layer formed on the metal substrate, and further a toner release layer of a fluorine-containing resin formed directly or via an adhesive layer thereon. In the commercial devices, the fluorine-containing resin layer has been given as a coating formed of a tube of fluorine-containing resin or formed by applying and baking a paint of fluorine-containing resins. In order to ensure a sufficient contact area between the recording material and the heating member, the pressure roller is required to exhibit a sufficient elasticity so that the elastic layer is formed in a relatively large thickness.
In recent years, not only in the above-mentioned pressure roller-drive type film-heating fixing device, but also in the hot roller fixing device and the fixing film-drive type film-hating fixing device, a higher-speed operation is strongly desired and also the adaptability to providing a smaller-size image forming apparatus is demanded. Further, for realizing economization of electricity, the demand for a heat-fixing device exhibiting an improved heat efficiency without requiring a standby temperature control is also increasing.
For complying with such demands for fixing devices, it is inevitably required to use a fixing roller and a pressure roller of relatively smaller diameters and operated at a relatively low pressure. Accordingly, in the case of an image forming apparatus operated at a high recording material-conveying speed, it becomes necessary to provide a broader pressure nip between the fixing roller or fixing film and the pressure roller at a low pressure in order to supply a sufficient heat to the recording material. For this reason, it has been tried to develop a pressure roller having a lower roller hardness.
For example, as a low-hardness fixing pressure roller, JP-A 7-271233 has proposed a fixing pressure roller comprising a silicone rubber layer formed from liquid silicone and having hollow pores continuous in its longitudinal direction, and also a fluorine-containing resin layer formed on the silicone rubber layer and having a surface hardness of at most 60 deg. as measured by an Asker-C hardness meter (1 kg-f).
JP-A 7-271233 has proposed a pressure roller having an elastic layer comprising a porous silicone rubber sponge layer having a hardness of 35-50 deg. (Asker-C) and a tube of PFA (tetrafluoroethyleneperfluoroalkyl vinyl ether copolymer) coating the sponge layer, thus providing an entire roller hardness (Asker-C) of 45-60 deg. so as to exhibit a good fixing performance while preventing the occurrence of paper wrinkles.
The above-mentioned JP-A 63-313182 and JP-A 2-157878 disclose a film-heating fixing device including a heating unit comprising at least a fixedly supported heating member (heater) and a cylindrical heat-resistant film for fixation conveyed while being pressed against the heating member, and a pressing member for pressing a recording material against the heating member, so as to transmit a heat from the heating member to the recording material via the film, thereby heat-fixing a yet-unfixed toner image carried on the recording material onto the recording material surface.
An example of such a heat-fixing device is illustrated in FIG. 3. Referring to FIG. 3, the fixing device includes a cylindrical film 101 comprising a 40 to 60 xcexcm-thick polyimide base film and a 40 to 60 xcexcm-thick release layer of PFA (tetrafluoroethylene-alkyl vinyl ether copolymer) alone or in mixture with PTFE (polytetrafluoroethylene) dispersed in PFA formed thereon for constituting an outer layer contacting a recording material P and a toner image T thereon.
The fixing device also includes a heating member (heater) 102, which basically comprises an insulating, heat-resistant and low-heat capacity ceramic substrate extends in a longitudinal direction perpendicular to the conveyance direction for the recording material P and a resistive heating member printed in the longitudinal direction on the ceramic substrate, and a temperature-detecting device 103 (of a thermistor, etc.) disposed in contact with and on a surface of the substrate opposite to the exposed resistive heating member.
The heater 102 is fixedly supported by a film guide (heater stay) 104 having an arcuate section so as to expose its heating member toward the film 101 and be otherwise insulated. The temperature of the heater 102 is controlled by controlling a current supply to the resistive heating member from a power supply (not shown) depending on an output from the temperature-detecting device 103.
The fixing device further includes a reverse U-shaped reinforcing metal sheet 108 for preventing deformation of the heater unit including the heater 102, the thermistor 103, the film guide 104, etc., under a pressure exerted by the pressure roller 105. The film 101 is designed to have an inner circumferential length larger than the peripheral length of the film guide 104 and the reinforcing metal sheet 108.
The pressure roller 105 is composed of a core metal 106 and a heat-resistant rubber layer 107 and is pressed against the heater 102 at a total pressure of 5-15 kg-f by being supported by a supporting means (not shown). The pressure roller 105 is driven in rotation in a counterclockwise direction by a drive means (not shown), whereby the film 101 is moved relative to and in intimate contact with the heater 102 and rotated in a clockwise direction around the film guide 104. In this instance, heat-resistant grease is caused to be present between the heater 102 and the film 101 so as to reduce the friction between these members.
As a result, a recording material P is guided to between the film 101 and the pressure roller 105 to be pressed through a nip N therebetween, whereby a toner image T on the recording material P is heat-fixed onto the recording material.
The above-mentioned film-heating fixing device is allowed to have a heat capacity of the heater unit which is one several tenth that of the conventional hot roller fixing device, and also a heat-generating member capable of quick temperature rise, so that the heating member can reach a fixing temperature within several seconds to ten and several seconds. Accordingly, it becomes possible to effect the so-called on-demand fixation that has been difficult to realize by the hot-roller fixing device.
In the film-heating fixing device, a thermistor 103 is abutted onto a side of the heater 102 opposite to the side provided with a printed resistive heat-generating member to detect an abnormal temperature rise of the heater 102, and data therefrom is transferred to a control means so as to control a switching device (not shown) to interrupt a current supply to the heating member.
However, in some cases of using a conventional pressure roller in such a film heating-type fixing device to operate the fixing device intermittently in a low temperature environment, the pressure roller surface is liable to be soiled with the toner to frequently result in difficulties, such as disorder of fixed images, paper wrinkles and paper winding about the pressure roller.
The toner solid on the pressure roller is observed to be initiated by attachment of paper dust which functions as nuclei for accumulation of the toner transferred from the fixing film. It has been confirmed that the paper dust attachment is more frequently caused when the pressure roller is driven at a lower surface temperature. In a heat-fixing device of electricity economization-type having no standby temperature control means, the pressure roller surface is caused to contact transfer paper (recording material) before the surface is warmed, so that paper dust attachment is liable to occur.
The paper dust attachment at a lower pressure roller surface temperature is also provided by a mechanism as follows. Paper dust is also attached onto toner already attached onto the pressure roller surface. In this instance, if the pressure roller surface is sufficiently heated, the toner is softened and is attached to transfer paper conveyed thereto so that paper dust can be removed easily together with the toner. However, if the pressure roller surface is not sufficiently heated, it is difficult to effect paper dust removal according to such mechanism.
Further, as mentioned above, in the film heating-type fixing device expected to perform electricity economization and shorter waiting time (i.e., improved quick-start characteristic), the heating member (fixing film) has a small heat capacity so that the pressure roller cannot be easily heated, and also in view of a demand for a smaller-size image forming apparatus, it is required to use a fixing film and a pressure roller of small diameters operated at a low pressure, so that it becomes difficult to obviate electrostatic offset. Particularly during an intermittent operation in a low temperature environment, the amount of toner offset to the fixing film is liable to be increased. This is presumably because of the following mechanism.
Toner transferred and attached to transfer paper is subject to an electrostatic force of attraction onto the fixing film. The electrostatic attraction force is for example caused by transfer of a charge of the transfer paper for attracting the toner to the fixing film surface. In case where the pressure roller surface is sufficiently heated, the toner on the transfer paper is heated not only by heat from the fixing film but also by heat from the pressure roller via the transfer paper, so that the toner on the transfer paper is sufficiently softened to generate a viscous adhesive force, by which the offset of the toner onto the fixing film is effectively prevented. However, in case where the pressure roller surface is not sufficiently heated, heat supply from the pressure roller is scarce and the toner is not sufficiently softened to generate a viscous adhesive force, so that a portion of the toner on the transfer paper can be electrostatically offset onto the fixing film surface. The offset toner onto the fixing film can be transferred to and accumulated on the pressure roller, thus being liable to cause difficulties, such as fixed image disorder, paper wrinkles and paper winding about the pressure roller.
The conventional pressure rollers have been developed to have a lower hardness of the entire roller so as to ensure fixing and conveying performance, and have been unsatisfactory for obviating paper dust attachment onto the roller surface.
Further, many of the fixing devices developed heretofore are not equipped with a cleaning device therefor for the propose of cost reduction, etc. Further, the use of smaller particle size toner for providing higher quality images in recent years makes difficult the cleaning by such cleaning means and has promoted the paper dust attachment. On the other hand, for the purpose of improving storage stability for a long period of paper, acidic paper has been gradually replaced by neutral paper, and accompanying therewith, calcium carbonate is being popularly used as paper filler. Further, a larger percentage of recently produced paper inclusive of regenerated paper tends to contain plural species of inorganic materials, such as calcium carbonate and talc, as paper fillers.
Such a filler is however liable to be attached onto the pressure roller, the fixing roller or fixing film, etc., thereby lowering the releasability of these members, so that the toner is liable to be accumulated on the pressure roller to result in spotty defects in the resultant images, and a difficulty such as paper winding about the pressure roller, leading to paper jamming, is liable to be caused.
A principal object of the present invention is to provide a pressure roller for use in a heat-fixing device of an electrophotographic image forming apparatus having solved the above-mentioned problems.
A more specific object of the present invention is to provide a pressure roller exhibiting stable releasability free from surface soiling for a long period even in an intermittent operation in a low temperature environment, thus exhibiting long-term durability and appropriate degree of gripping performance so as to retain good recording material conveying performance.
Another object of the present invention is to provide a pressure roller satisfying required fixing performance and durability and also capable of effectively suppressing toner sticking onto the roller surface, thus obviating difficulties, such as image soiling and paper jamming.
A further object of the present invention is to provide a heat-fixing device including such a pressure roller and exhibiting a high reliability in a long term use.
According to the present invention, there is provided a fluorine-containing resin-coated pressure roller, comprising a cylindrical metal substrate, and a rubber elastic layer, an adhesive layer and a fluorine-containing resin release layer disposed in this order on and successively coating the cylindrical metal substrate; wherein the pressure roller has a surface exhibiting a micro-rubber hardness of at most 50 deg., a contact angle with water of at least 105 deg. and a ten point-average surface roughness Rz of at most 3.0 xcexcm.
If the pressure roller has a surface exhibiting such a low hardness and rich in softness, it is considered that the roller surface can repetitively cause circumferential elongation and contraction of the nip and in the neighborhood thereof, so that paper dust is less liable to attach to the roller surface and paper dust once attached to the roller surface due to static electricity, etc., can be released therefrom. Accordingly, it becomes possible to prevent toner soiling on the pressure roller surface caused by attached paper dust. On the other hand, if the pressure roller surface has a micro-rubber hardness exceeding 50 deg., it becomes difficult to attain the above effect due to insufficient circumferential elongation and contraction of the pressure roller surface at and in the neighborhood of the nip.
Further, by setting the pressure roller surface to exhibit a contact angle with water of at least 105 deg., the pressure roller surface is caused to have a lower surface energy, thereby reducing the attachment of paper dust and toner.
Further, by setting the pressure roller surface to have a roughness Rz of at most 3.0 xcexcm, it becomes possible to prevent the intrusion of toner into concavities of the roller surface leading to formation of nuclei of further toner and paper dust attachment.
As a result of synergistic combination of the above-mentioned effects attributable to prescribed physical properties, the pressure roller of the present invention is believed to exhibit good conveying performance without toner attachment.
According to another aspect of the present invention, there is provided a heat-pressure fixing deice including the above-mentioned fixing device.
The pressure roller of the present invention is particularly effectively used in a film-heating type fixing device. Thus, according to still another aspect of the present invention, there is provided a film heating-type fixing device for use in an electrophotographic image forming apparatus, comprising: a fluorine-containing resin-coated elastic pressure roller and a fixing film having a surface release layer disposed opposite to the pressure roller surface, wherein
the surface release layer of the fixing film comprises a fluorine-containing resin and exhibits a ten point-average surface roughness Rz of at most 5 xcexcm; and
the fluorine-containing resin-coated pressure roller comprises a cylindrical metal substrate, and a roller elastic layer, an adhesive layer and a fluorine-containing resin release layer disposed in this order on and successively coating the cylindrical metal substrate; wherein the pressure roller has a surface exhibiting a micro-rubber hardness of at most 50 deg., a contact angle with water of at least 105 deg. and a ten point-average surface roughness Rz of at most 3.0 xcexcm.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.